This invention relates to a rotary encoder capable of being positioned at a required position with high accuracy, and to a method of manufacturing the rotary encoder.
Along with the advances that have recently been made in the field of robots and automatic machines, encoders which serve as devices for detecting rotational and linear positions have come into wide use for the purpose of controlling those robots and machines. A great variety of such encoders are available. Incremental rotary encoders are the most commonly employed rotary encoders for measuring the displacement of a rotational angle. Such an encoder will now be described.
Rotary encoders presently available on the market are of two types, namely optical and magnetic. However, recent progress in the field of digital technology has produced a need for rotary encoders having high resolution and reliability.
For example, in the magnetic-type encoder, magnetic graduations indicative of position information are written into a rotary disk and the graduations are read by a magnetic head, which responds by outputting signals of the kind shown at X, Y and Z in FIG. 5, whereby position is detected. It should be noted that the left-hand portion of FIG. 5 is a timing chart for forward rotation, and the right-hand portion of FIG. 5 is a timing chart for reverse rotation. The direction of rotation can be identified by comparing the two signals X and Y. When the signal Z is outputted, this indicates that the rotary disk is at a one-revolution initial position.
However, in order to obtain a high resolution with a disk of the same shape and size, the pitch of the signal generating sources generally must be reduced. This leads to a weakening of the generated signals and a deterioration in the S/N ratio, resulting in a decline in reliability. In addition, a high processing speed is required and the associated control circuitry is complicated and costly. The processing speed can be lowered by reducing the number of output pulses per revolution, resolution declines and positioning precision suffers.
By way of example, consider a rotary encoder which generates 1000 pulses per revolution. If a motor on which such an encoder is mounted is a high-speed motor that rotates at 6000 rpm, the pulses outputted by the encoder will have a frequency of 100 KHz. By comparison, a rotary encoder having a resolution of 10,000 pulses per revolution will produce output pulses having a frequency of 1 MHz at the same rotational speed of 6000 rpm.
In order to use such a high-resolution encoder mounted on a high-speed motor as is strongly demanded, a high-performance computation control circuit capable of executing processing at a very high speed must be provided to process the output pulses from the encoder. Such processing is not possible with an ordinary microcomputer.
Moreover, no matter how much the pitch is reduced to raise resolution, the control position is at best an approximate position with respect to an ideal position and does not always agree with the required position information.
FIGS. 9 and 10 illustrate the construction of an optical disk used in a conventional optical rotary encoder of the type most widely employed in the art.
As shown in FIGS. 9 and 10, an optical disk 100 is fixedly secured on a drive shaft 110 which rotates in response to movement of an object under control, the shaft 110 passing through the center of the disk 100. The optical disk 100 is formed to include slits 101 in the thickness direction thereof through which a beam of light is allowed to pass, the light beam being blocked by the solid portions of the disk body between the slits. The slits 101, which extend radially of the disk 100, are arranged along the entire circumference of the disk at a prescribed radius from the center thereof and are angularly spaced at a predetermined angle .alpha.. The slits 101 and intervening solid portions need only transmit and block the light beam and can consist of alternately arranged transparent and opaque portions, respectively.
As shown in FIG. 10, the optical disk 100 is interposed between a light-emitting element 105, which produces the aforementioned light beam, and a light-receiving element 106 arranged to oppose each other at the portion of the disk where the slits 101 are provided. When the optical disk 100 rotates, the light beam from the light-emitting element 105 passes through the slits 101 to impinge upon the light-receiving element 106 and is blocked from the light-receiving element 106 by the solid portions of the disk 100 between the slits 101. Thus, the light-beam from the light-emitting element 105 is allowed to impinge upon the light-receiving element alternatingly. By thus sensing the light beam, the light-receiving element 105 makes it possible to detect the amount by which the optical disk 100 rotates.
In the case of a magnetic-type rotary encoder, magnetic graduations are recorded on a rotatable disc in advance by applying a highly coercive magnetic material to the disc at positions corresponding to the circumferential positions at which the slits 101 of optical disc 100, shown in FIG. 9, are located, or by applying the magnetic material to the peripheral surface of the disc at the abovementioned positions. These magnetically recorded graduations are read by a magnetic head arranged at a predetermined position opposing the circumferential edge portion of the disc or at a predetermined position opposing the peripheral surface of the disc. The amount of disc rotation is detected by e.g. counting the number of pulses produced in accordance with the amount of disc rotation.
However, the spacing between adjacent slits in the optical system or between the adjacent magnetic graduations in the magnetic system is a certain fixed angle, denoted by .alpha. in FIG. 9, and the accuracy with which an angular position can be expressed is decided by this angle .alpha.. In other words, expressing a position within the angle .alpha. (i.e. obtaining a position detection accuracy smaller than .alpha.) is impossible. In order to improve accuracy, the slit pitch must be reduced (i.e. the number of slits must be increased) to make .alpha. smaller.
Mechanically speaking, this demands higher machining and assembly precision. Electrically speaking, reducing the size of the slits results in less light passing through the slits and worsens the S/N ratio. Consider a case where an optical-type incremental rotary encoder is used, position is detected at a point where the encoder shaft rotates through 90.degree. and rotation is stopped at this point. If the encoder outputs 400 pulses per revolution, then rotation should be stopped at a position detected to be 100 pulses from a reference point. However, only the 100th pulse is actually needed, the 1st through 99th pulses not being directly required. Nevertheless, all of these pulses must be constantly monitored in the conventional arrangement. Furthermore, if it is attempted to stop rotation at the 89.degree. position and not the 90.degree. position, this cannot be accomplished because there is no slit at the 89.degree. position. Thus, stopping rotation at an arbitrary position is impossible.